Techniques have been developed for reducing the noise emitted by an aircraft and in particular the noise emitted by a propulsion system, by using, at the walls of the pipes, panels (also called coatings or structures) whose purpose is to absorb a portion of the sound energy, in particular by using the principle of Helmholtz resonators. In a known manner, an acoustic treatment panel comprises—from the outside to the inside—an acoustically resistive porous layer, at least one alveolar structure, and a reflective or impermeable layer.
Layer is defined as one or more layers that may or may not be of the same type.
The acoustically resistive porous layer is a porous structure that plays a dissipative role, partially transforming the acoustic energy from the sound wave that passes through it into heat. It comprises so-called open zones that are able to allow acoustic waves to pass and other so-called closed or filled zones that do not allow sound waves to pass but are intended to ensure the mechanical resistance of said layer. This acoustically resistive layer is characterized in particular by an open surface ratio that essentially varies based on the engine and components constituting said layer.
The alveolar structure is delimited by a first imaginary surface to which the acoustically resistive porous layer can be connected directly or indirectly and by a second imaginary surface to which the reflective layer can be connected directly or indirectly, and said structure comprises a number of pipes that empty, on the one hand, at the level of the first surface, and, on the other hand, at the level of the second surface. These pipes are blocked by, on the one hand, the acoustically resistive porous layer, and, on the other hand, the reflective layer in such a way as to form a cell.
A honeycomb can be used to form the alveolar structure. Different types of materials can be used for forming the honeycomb.
When the acoustic panel is installed at an air intake of a nacelle, this acoustic panel is also to be compatible with a frost treatment. Frost treatment is defined as a process or a system that makes it possible to prevent the formation and/or the accumulation of ice and/or frost.
The documents EP-1,232,944 and EP-1,232,945 describe panels for the acoustic treatment that is compatible with a frost treatment that uses hot air. In this case, the alveolar structure comes in the form of bands of cells spaced apart or a number of pipes spaced apart.
This frost treatment is generally coupled to a drainage system that makes it possible to remove the water that can accumulate in the cells of the alveolar structure, in particular in the cells that are located in a zone that extends from 3 o'clock to 9 o'clock.
This drainage of the water is necessary for limiting the risks of deterioration of the cells of the alveolar structure due to the gel of the accumulated water, for preventing corrosion problems, and for maintaining the acoustic performance levels.
According to one embodiment, the side walls of the cells comprise cutaways in the upper portion or in the lower portion in such a way as to link the cells to one another, with the cells located at 6 o'clock comprising openings at the reflective layer in such a way as to remove the water from the acoustic treatment panel.
This embodiment is not completely satisfactory because the presence of a drainage system coupled to a frost treatment system with hot air tends to cancel out the acoustic treatment and to generate a stream that is disrupted at the stream of air entering the nacelle.
Also, the purpose of this invention is to remedy the drawbacks of the prior art by proposing an acoustic treatment panel incorporating a frost treatment and a drainage system optimizing the yield of the three functions, namely acoustic treatment, frost treatment, and drainage.